User input means (such as a user interface) of an electronic device can be implemented in various ways. Touch pads, keyboards, keymats, touch-screen, etc. are well known user interfaces especially for portable devices as laptop computers and mobile telephones. A touch pad is an input device which typically includes a sensor and an associate circuitry. When a user moves a stylus or a finger to touch (or to put in a close proximity) the touch pad, that contact effects the sensor and is detected by the circuitry. There are various mechanisms for detecting the point of contact on the touch pad.
One approach for detecting a user input is generating an electrical field and detecting a deformation of the electric field by a user. The electric field can be generated, for instance, within the area of a touch-screen. The disturbance of that field caused by the object may then depend on the position at which the touch-screen is touched by the object (e.g., stylus, finger of the user, etc.). For generating and monitoring such an electrical field, different sensor technologies can be employed. One option is to use a capacitive detection.
Among multiple capacitive touch pad principles, a capacitive detector can comprise at least one conductive plate or electrode (element X), which forms a capacitance with at least one another conductive plate or electrode (element Y) as shown in FIG. 1. In this capacitive detector, an electric filed is set between these electrodes. Then the disturbances of the electric field induced, for example, by a user finger (e.g., by touching, which can act as grounding or disturbing element) can be detected by monitoring the capacitance value between elements X and Y as shown in FIG. 1. Thus capacitance values (i.e., changes in the disturbed electric field) can be used for detecting whether there is some object in close vicinity of the detector or not, and at which position. This principle can be used in a matrix type grid sensor arrangement with rx and tx electrodes separated by a gap, wherein the object (e.g., a finger) causes disturbances in coupling the signal which is detected by the measurement circuitry, as disclosed, for example, in U.S. Pat. No. 6,452,514 “Capacitive Sensor and Array” by H. Philipp.
There are other multiple alternative methods and variations in the measurement technique in using the capacitance measurement for detection. For example, principles, disclosed in U.S. Pat. No. 6,466,036 “Charge Transfer Capacitance Measurement Circuit” by H. Philipp, can be applied to semi-conductive plate (or possibly to a conductive plate) to measure the location of the finger as well, using the following. Charge pulses can be injected from a number of electrodes placed around the touch plane (e.g., semiconducting touch plane) at least three preferably at least four electrodes. There can be more electrodes for increased accuracy and performance. These charge pulses generate electric field around the semi-conductive plane and the finger absorbs energy of some of the pulses (capacitive connection to the plane). The injected charges are collected and counted. The sensing electrodes from the corners of the touch plane have resistance values to the point which forms the capacitance connection to the finger, i.e., changes in the resistance can be detected as changes in an electric current (resistive-capacitive detection). Relative resistance values determine the distances from the corners indicating coordinate values.
The finger (or another object) can interfere with the measurement arrangement by acting, for example, as a grounding element (e.g., by absorbing energy from the electric field). Thus the touch signal sensing strength can be dependent on the distance of the finger from the sensing element. In many cases the overlaying materials and structures on the touch pad are not flat which can cause uneven capacitive sensor signals. In other words, in case of applications with non-flat touch pads, it is difficult to set a certain capacitive value as a threshold value for a touch indication, because this touch capacitive value is different between locations where the finger touches on the touching pad. This can generate signal difference for different locations on the touch pad which may cause nonlinearity in analog capacitive touch sensing.